The development of affordable photovoltaic technologies, i.e., solar cells, offers one of the promising solutions for clean, renewable energy. Solution-processed polymer photovoltaic cells have attracted much attention since they can be manufactured with inexpensive, high-throughput techniques [1]. They are light weight and can also be flexible, making it possible to integrate them with ordinary surfaces at a large scale to provide inexpensive power sources. They can also be customized to serve as cheap, disposable, soft generators for remote, autonomous micro devices. With a rapid progress in both materials design and device architectures, it is envisioned that polymer photovoltaic system with power conversion efficiency (PCE) over 10% can be realized [2].
In a typical polymer solar cell, the transparent anode, e.g., indium tin oxide (ITO), is usually coated with a modifying layer that can block electrons but help to transport holes to minimize carrier recombination. Among the successful hole transporting layer materials, the conjugated polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been predominantly used as an anode modifier due to its exceptional solution processability [1]. Li et al. first reported that graphene oxide (GO) can also be used as a hole transporting layer on ITO for polymer solar cells, potentially replacing PEDOT:PSS [3]. GO is a graphene derivative decorated with oxygen-containing groups such as carboxylic acids, epoxides and hydroxides that can be readily synthesized by oxidative exfoliation of graphite powders to yield stable aqueous dispersion of single layer sheets [4-6]. The apparent thickness of a GO sheet was measured to be around 1 nm by atomic force microcopy (AFM) but its lateral dimension can readily extend to tens of microns [7, 8]. The large band gap of GO helps block electron flows towards the anode, thus effectively suppressing the carrier recombination [3, 9]. The performance of the GO hole transporting layer was demonstrated to be comparable to PEDOT:PSS. However, since GO is insulating, it also increases the internal resistance of the device, which could lower the fill factor unless its thickness can be kept at the minimum, ideally down to one monolayer. An earlier work of the inventors has shown that GO pieces can be tiled up by Langmuir-Blodgett assembly to obtain high coverage monolayers [7, 10]. Since spin coating is routinely used for fabricating polymer solar cells, it would be a more desirable method if very thin GO modifying layers can be made. Li et al. used spin coating to make the GO modifying layers on ITO surface and found that only when the thickness of the GO films was reduced to around 2 nm, were their performances comparable to PEDOT:PSS [3]. However, spin coating such thin GO films (i.e., one to two monolayers) can be extremely challenging because the resulting films also need to fully coverage the underlying surface.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.